As far as I know, there aren't any Cydia apps which support iMessage - although I could well be wrong. As a long-time iPhone user and having tried jailbreaking, though, my advice would be against that route. If you check your phone periodically the above will work, otherwise submit a bug req to Apple - they are very good at listening to what people want and may well sort the problem out for you. Also possible: does the other iMessage user have it setup with an email as well? You could iMessage that = no failures (in theory) and no fallback, and then in emergency use their number.
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George PearceJan 30 '12 at 18:15